Researchers in the US have built a new optical clock from strontium atoms that is accurate to about one part in 1016 — meaning it would neither gain nor lose a second in more than 200 million years. While the clock is not the world’s most accurate — that honour goes to an optical clock based on a single mercury ion, which is accurate to several parts in 1017 — the strontium device is the world’s most accurate clock to use neutral atoms.

The clock, which was built by Jun Ye and colleagues at JILA and the University of Colorado in Boulder, is based on thousands of strontium atoms that are trapped in an "optical lattice" made from overlapping infrared laser beams (Sciencexpress). The atoms are bathed in light from a separate red laser at a frequency corresponding to an atomic transition in strontium — which causes this light to lock into the precise frequency of the transition. The oscillation of this light is just like the ticking of a clock — the first neutral-atom timekepeer to be more accurate than the standard caesium-fountain atomic clock.

3.5-km optical fibre

The team determined the clock's accuracy by sending its time signal via a 3.5-km underground optical fibre from JILA to the National Institute of Standards and Technology (NIST) in Boulder, where the signal was compared to that from an optical clock based on neutral calcium atoms. This is the first time that researchers have been able to compare signals from two optical clocks separated by several kilometres in this way.

The time signals from the clocks were compared using two “frequency combs” located at either end of the fibre link. Each clock produces light at different frequencies and the combs are used to convert the signals to a lower common frequency. After being shifted to the lower frequency, the strontium signal was amplified using a laser and then transmitted to NIST.

Adding anti-noise

The two signals were then combined and the physicists looked for signs of interference — or “beating” — which occurs if the time signals have slightly different frequencies. Ye told physicsworld.com that a key challenge in transmitting the signal was to ensure that it was not disrupted by noise in the optical fibre. This was done by monitoring the noise in the fibre and introducing an “anti-noise” signal to cancel it out.

The team is now trying to improve the precision of the clock — a measure of how reliable its time signal is — by increasing the number of atoms used. The team also plan to use their technique to compare the strontium clock to a mercury-ion clock at NIST.

New standard

Most physicists believe that an optical clock will someday replace the caesium-fountain atomic clock as the time standard. That is because the stability of an atomic clock is proportional to its operating frequency, which means that clocks based on narrow transitions at optical frequencies will be much more stable than those based on much lower frequency microwave transitions. Clocks need to be both stable and accurate, with greater stability making it easier and faster to work out how accurate a clock really is.

However, several different optical clocks being developed around the world and no clear frontrunner has emerged. The best single-ion clocks are currently more accurate than their neutral-atom counterparts , but their signal comes from just one ion, making them noisier and perhaps less practical than neutral-atom clocks.

Patrick Gill of the UK’s National Physical Laboratory, which is also developing optical clocks, described JILA’s strontium clock as an “impressive step on the way [to an optical standard]”, but added that it “may not be a clear winner". According to Gill, while strontium is a popular candidate for a next-generation time standard, there are about a dozen candidates for the job — both neutral atoms and ions.